We plan the following multidisciplinary approach to study the prognostic significance of "late potentials" in the body surface electrocardiogram (ECG) as an indicator of ventricular electrical instability: 1) We will continue our studies to develop a low noise system by a combination of spatial averaging with optimal interelectrode distance and a volume conductor recording. 2) We will investigate a novel approach by which periodicity of cardiac potentials generated from ischemic canine myocardium (Wenckebach type and 2:1 periodicity) could be detected on the body surface as sub-harmonics by analysis of the frequency domain of the ECG signal utilizing Fast Fourier Transform Analysis (FFTA) techniques. 3) We will continue our studies in various canine post infarction models to determine to what extent delayed activation potentials measured by direct ventricular recordings can be detected on the body surface by signal averaged ECG and spatially averaged low noise ECG. The effects of antiarrhythmic drugs and of cryothermal ablation on the activation patterns of reentrant rhythms will be correlated with changes in the late potentials recorded on the body surface. 4) We will continue our clinical studies to determine whether one or more of several techniques for recording and analysis of late potentials could provide information useful in identifying patients with ventricular electrical instability that is independent from, and possibly superior to, that which can be obtained from other tests or a combination of tests. In post-infarction patients and in patients with spontaneous or electrically induced ventricular arrhythmias, sequential recordings of late potentials will be obtained utilizing both signal averaged ECG and spatially averaged low noise ECG. Spectral analysis of late potentials for periodicity will also be performed. A stepwise logistic regression will be utilized to determine the prognostic significance of late potentials as compared to long-term ambulatory ECG recordings, programmed electrical stimulation and specific clinical and anatomic indices. We will investigate whether changes in late potentials induced by antiarrhythmic agents and/or antiarrhythmic surgical techniques could be used to predict the outcome of these therapeutic measures. Finally, we will utilize our recently described technique of signal averaging of high gain holter ECG recordings to investigate the possibility that dynamic changes of late potentials occur prior to the onset of spontaneous tachyarrhythmias that may not be present otherwise during arrhythmia-free periods.